London: British Prime Minister David Cameron has condemned the killing of another journalist by Islamic State extremists as "a despicable and barbaric murder".

A video of the latest killing reported to be US journalist Steven Sotloff emerged on Tuesday afternoon UK-time, featuring the same militant who killed James Foley, named in the British media as British jihadist Abdel-Majed Abdel Bary, dubbed "Jihadi John".

According to intelligence group SITE, the killer also threatened another captive, thought to be a British aid worker.

Advertisement

The Washington Post reported that the man was shown in the video in the same kneeling position and wearing the same orange jumpsuit as Sotloff and Foley before him.

He was abducted in March 2013 near the Atmeh refugee camp adjoining the Turkish border in the northern Syrian province of Idlib, aid workers who have been involved in efforts to secure his release said.

Mr Cameron said he would chair a meeting of the government's Cobra emergency committee meeting on Wednesday morning.

NBC identified the captive as a former British soldier who worked for a string of aid organisations. However, a UK Foreign Office spokesman said they were working to verify the authenticity of the video.

A spokeswoman for civilian peacekeeping group Nonviolent Peaceforce told US network NBC the man in the video did security work for them in South Sudan in 2012 and was working for another aid group when he was reported abducted in early 2013 in Syria.

"A fellow he was abducted with was released a couple of months ago, and we were hopeful that he would be released soon," Tiffany Easthom, the South Sudan country director for the organisation, told NBC.

When she heard he had been threatened in the video "my heart went into my throat".

She said the man was "ex-British military and very familiar with insecure locations ... He was very caring, had a good sense of humour. He left the military and wanted to work for NGOs."